Method of producing tungsten



Patented Mar. 1, 1949 F F I C E METHOD OF PRODUCING TUNGSTEN Colin G.Fink and Ohuk Ching Ma,

New York, N. Y.

No Drawing. Application February 19, 1944, Serial No. 523,092

This invention relates to the recovery of metallic tungsten fromtungsten compounds, tungsten ores and concentrates of tungsten ores.More specifically, the invention provides an electrolytic method for thedirect production of crystalline grains of substantially pure metallictungsten from compounds, ores, and ore concentrates of tungsten.

Tungsten occupies a position of major importance among industrialmetals, owing to its widespread use not only as lamp filaments but alsoas a constituent of alloy steels, of cutting tools, of dies, and otherarticles which must resist abrasion or be strong at high temperatures.

The commercially important ores of tungsten are scheelite (calciumtungstate) and Wolframite, an iron-manganese tungstate of which twocommon variants are the iron-rich ferberite and the manganese-richhuebnerite. These ores are usually concentrated by hand or mechanically,although recently chemical extraction methods have been developed forleaching lean ores of tungsten and producing concentrates in the form ofpure or nearly pure calcium or sodium tungstate. The concentrates,however produced, are ordinarily either smelted in the electric furnaceto produce a ferrotungsten alloy, or purified and converted to tungsticoxide which is then reduced with carbon or hydrogen to pure or nearlypure tungsten metal.

It has been proposed to produce metallic tungsten by electrolyticreduction of tungsten compounds. in 1867 Zettnow (Pogg. Ann, vol. 130,pages 16 and 241) described the production of an impure tungsten uponelectrolysis of sodium tungstate. In 1919 Keyes patented (Patent1,293,117) a method of producing finely divided tungsten metal byelectrolyzing at 1200 to 1400 C. a solution of tungsten oxide in boricacid. In 1924 Kahlenberg and Kahlenberg (Trans. Electrochem. Soc, vol.46, page 181) described the preparation of a smooth plating of tungstenby electrolyzing at low current densities an electrolyte formed byheating tungstic oxide with fused sodium chloride. In 1925 Van Liempt(Zeitschr. Elektrochem., vol. 31, page 249) disclosed the production ofamorphous tungsten powder by electrolyzing alkali metal tungstate. In1929 Andrieux (Ann. Chim., vol. 12, page 495; Comptes Rend, vol. 184,page 91), having found that fused mixtures of tungstic and boricanhydrides form very viscous baths havin poor conductivity, describedthe production of a crystalline tungsten powder by electrolyzing a fusedmixture of tungstic oxide, borates, and fluorides. Andrieux consideredthe further addition of zinc oxide to be necessary to obtain puretungsten at a high current efiiciency. In 1931, Hartmann et a1.(Zeitschr. Anorg. Chem., vol. .198, p. 116) described a method ofproduc- 3 Claims. (01. 204-) fused mixture of sodium pyrophosphate andmetaphosphate with tungstic acid, at 650-700 C., and also from asolution of tungstic oxide in sodium pyrophosphate. This process wasfurther investigated by Leo and Shen (Trans. Electrochem. Soc., vol. 66,p. 461).

It will be noted that all of these prior investigators used eithertungstic oxide or sodium tungstate, a water-soluble salt. Apparently,none considered his method applicable to impure tungsten materials suchas ores or concentrates. Inasmuch as relatively expensive procedures arerequired to produce pure tungstic oxide or sodium tungstate fromtungsten ores, these electrolytic reduction methods offer little or nocost advantage over other reduction methods in producing tungsten,particularly when it is considered that prior workers were able tooperate only at low current efiiciencies and were unable to prevent thesimultaneous production of large amounts of compounds of lower oxides oftungsten or tungsten bronze. Largely for these reasons, production oftungsten metal by electrolysis has not attained commercial acceptance.

It has now been discovered that tungsten ores and concentrates, such aswolframite, ferberite,

. 6 ing metallic tungsten electrolytically from a huebnerite, andscheelite, preferably in finely granular form, may be readily dissolvedin molten alkali tetraborates, and that under certain conditionsdescribed hereinafter the solutions may be electrolyzed to producesubstantially pure, crystalline tungsten metal grains.

More specifically, the invention comprises mixing a tungsten ore or atungsten ore concentrate with molten sodium tetraborate (NazB4O7),maintaining the solution so obtained at a temperature between 950 and1300" C., and electrolyzing said hot solution between an anode and acathode at a cathode current density between 2 and 4 amperes per squareinch. Neither fluorides nor zinc oxides are used in the salt bath.

It is preferred that the initial proportions of ingredients be such thatthere are between 1.5 and 3.5 parts of solvent to each part by weight oftungstic oxide (although as much as '7 parts of solvent to 1 part oftungstic oxide may be used), and it is also preferred that theproportions be maintained within these limits throughout theelectrolysis. Gangue constituents may becloud the solution, but suchcloudiness does not interfere with the process.

Tests have established that the solvent sodium salts may be replacedwholly or in part by the corresponding potassium salts withoutsubstantially changing the characteristics of the process. It is alsopreferred that the temperature of the solution during electrolysis bemaintained between 1050 C. and 1250 C.

The preferred current density range is from 2.5 to 3.5 amperes persquare inch ofv cathode.

current density at the anode is not critical but is preferably of aboutthe same order as that at the cathode.

The cathode may be made of any suitable conductive material, such asiron or graphite; but is preferably of molybdenum or tungsten. Graphiteis the preferred anode material.

The process steps and conditions justdescribe'd are critical if a pureproduct is to be obtained under efficient operating conditions. Thus,baths initially containing less than about 28%, or more than about 67%by weight of tungstic oxide are electrolyzed only at very low currentefficiency. The temperature and the current density exert strong effectsnot only on the current efficiency but also on the purity of theproduct. It seems most probable, on the basis of extensive dataaccumulated in the course of investigation of this process, that a filmof sodium vapor is formed on the cathode, that for optimum results in'respect to current efficiency and purity of product there is a narrowrange of optimum thicknesses of such film, and that the most si nificantof the factors controlling such film thickness are thetemperature of theelectrolyte and the current density at the cathode.

There is a very sharp enhancement of current efficiency as theelectrolyte temperature is raised from 950 to 1025 C. A further increasein temperature is accompanied by a much less rapid increase in currentefficiency, until a maximum is reached in the range of 1050 to 1150 C.,after which, at higher temperatures, the efliciency decreases, againreaching a low value at about I300'-1350 CL Similarly,current efliciencyis at a maximum when the current density at the cathode is in theneighborhood of 3 amperes per square inch, and reaches a low value atdensities below 1.5 and above 4' amperes per square inch. The greatestpurity of product, i. e. greatest freedom from tungsten boride andtungsten bronzes, is achieved at current'densities above 2.5 amperes persquare inch.

In" the course of a series of experiments on the method of thisinvention, using wolframite concentrates dissolved in fused borax, bestresults were obtained under the following conditions: one part of weightof ore concentrate (60%-'70% W03) in each 1.5 to 0.75 part of fusedborax as the initial electrolyte; electrolyte temperature between 1050"and 1300 0.; current density at the cathode, about 50 amperes per squaredecimeter (3.23 amp/sq. in). Under these conditions, currentefficiencies in the neighborhood of 78%, yields of about 0.92 pound perkilowatt-hour, and a product analyzing 99.57% tungsten, have beenattained.

The tungsten metal, as it is formed, usually drops to the bottom of theelectrolyte bath where it forms a sludge which is readily separated fromthe main body of the electrolyte by decantation. Ifa hollow molybdenumcathtode is used, tungsten crystals will adhere to it. The sludge alsocontains various impurities such as compounds of lower oxides oftungsten. After the sludge has'been cooled and solidified, the puretungsten is easily separated by leaching. The metal itselfis'substantially free from those impurities such as phosphorus, arsenic,sulfur, and tin, which are commonly'present in tungsten ores but whichimpair the value of tungsten metal for use in steels. The impuritiesmaking up'the greater part of the Y tungsten metal are iron matures.

All

The process may be applied not only to high grade ore concentrates butalso to low rade ores and concentrates, with some decrease in currentefficiency. Somewhat better current efficiencies have been attained withthe use of wolframite, ferberite, and huebnerite than with scheelite,but very good results are achieved even with scheelite.

In general, within the ranges of conditions specified herein, raisingthe current density tends to yield smaller crystals of tungsten, whileraising the temperature tends to yield larger crystals. Insolubleimpurities, such as iron oxide, also effect the crystal size, anincreased concentration of such impurities tending to decrease thecrystal size.

What is claimed is:

1. Method of producing substantially pure tungsten crystalline grainsdirectly from oxidic tungsten ores and concentrates thereof whichcomprises mixing said tungsten-containing material with molten sodiumtetraborate maintaining the solution so obtained at a temperaturebetween 950" and 1300 C.; and electrolyzing, at such temperatures, saidhot solution as an electrolyte between an anode and a cathode at acathode current density between 2 and 4 amperes per square inch.

2. Method of producing substantially pure tungsten crystalline grainsdirectly from oxidic tungsten ores and concentrates thereof whichcomprises mixing said tungsten-containing ma"- terial with molten boraxin the proportions of 1.5 to parts by weight of borax' to each part byweight of tungstic oxide; maintaining the solution so formed at atemperature between 950 and 13=30 C.; electrolyzing, at suchtemperature, said hot solution as an electrolyte between an anode andcathode at a cathode current density between 2.5 and 3.5 ampers persquare inch, thereby forming a tungsten and salt sludge; separating andcooling said sludge; and leaching the cooled sludge to recoversubstantially pure tungsten metal grains.

3. Method of producing substantially pure tungsten crystalline grainsdirectly from oxidic tungsten ores and concentrates thereof whichcomprises mixing said tungsten-containing material with molten sodiumtetraborate maintaining the solution so obtained at a temperaturebetween 1050 and 1250 C.; and electrolyzing, at such temperatures, saidhot solution as an electrolite between an anode and a cathode at acathode current density between 2.5 and 3.5 amperes per square inch.

COLIN G. FINK. CHUK CHING MA.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,545,582 Cobb July 14, 19251,874,090 Driggs Aug. 30, 1932 FOREIGN PATENTS Number Country Date552,669 France Jan. 25, 1923 5,132 Norway Jan. 4, 1897 OTHER REFERENCESComptes Rendus, vol. 53, p. 727 (1861). Transactions of theElectrochemical Society, vol. 66, pp461-469 (1934)

